The present invention relates to composite coatings, in particular such coatings comprising carbon and another metallic element. The present invention relates also to methods of making composite coatings, materials for making the coatings and substrates coated with the coatings.
Amorphous silicon-carbon alloys (axe2x88x92Si1.xCx) have attracted much recent attention not only due to the composition dependent variability of their optical band gap but also because of their important role as intermediate layers for the growth of diamond films on crystalline silicon and non-diamond substrate. Several attempts have been made to deposit (axe2x88x92Si1.xCx) films using existing thermal chemical vapour deposition (CVD) or plasma assisted CVD techniques, but these techniques involve high deposition temperatures which may destroy or damage many substrate materials. Also, known CVD techniques have used metal organic compounds, undesirable due to their toxicity.
It is known to deposit hard thin films, such as tetrahedral amorphous carbon (ta-C), using a filtered cathode arc (McKenzie et al 1991, Fallon et al 1993, Martin et al 1988). These ta-C films have interesting and useful properties, such as extreme hardness (xcx9c70 Gpa), thermal stability, high electrical resistivity, wide Tauc optical band gap (xcx9c2.5eV), smooth surface and low friction, and transparency in wide spectral range because of the high sp3 fraction of carbon atoms (up to 87%) in the film.
However, the high internal stress in the films can limit their applications, especially when it is desired to deposit a relatively thick film, as the film may flake away from the substrate.
In order to reduce the internal stress of ta-C films, and in an attempt to improve adhesion of thick films of this type, different modifications have been made, such as nitrogen incorporation into the films. However, whilst the internal stress can be reduced a little, this is not sufficient to enable significant increases in usable film thickness. In addition, there are disadvantages to incorporation of nitrogen into these films as so doing can harm many of the mechanical properties of the films.
Metal-containing diamond-like-carbon (DLC) materials are known potentially to have useful electrical and mechanical properties, wear resistance and friction (Dimigen et al 1987). It has been reported that such films containing certain low percentages of metals can have comparable wear resistance and friction coefficient with the a-C:H films, and may have better adhesion to the substrate. Introducing certain metal elements such as aluminum into the DLC films may reduce film stress, but only at the unacceptable expense of its mechanical properties, such as hardness and Young""s modulus.
It is therefore an object of the invention to provide composite coatings that solve or at least ameliorate the aforementioned problems. In particular it is an object of specific embodiments of the invention to provide composite coatings that exhibit reduced stress, thus enabling deposition of relatively thick coatings whilst retaining acceptable hardness.
Accordingly, the present invention provides, in a first aspect, a method of applying a coating to a substrate using a cathode arc source, comprising:
generating an arc between a cathode target and an anode of the source; and
depositing positive target ions on the substrate to form the coating,
wherein the coating is a composite of at least first and second elements and the target comprises said at least first and second elements.
Thus, the invention enables the production of composite coatings from targets used in a cathode arc that contain two or more coating components. It is an advantage of the method that composite films can easily be produced using the filtered cathode arc process, and without the need for introduction of gaseous compounds into the arc vacuum chamber. Composite films were previously made using, for example, a graphite target and hydrocarbon gas, SiH4 gas or a metal organic compound in vapour form introduced typically close to the substrate. The resultant films had high hydrogen content and suffered from poor mechanical properties. The method of the invention avoids the necessity for gaseous components and enables production of films that have lower hydrogen contents than and improved mechanical properties than possible hitherto. Films of the invention typically have a hydrogen content of 20% or less, preferably 10% or less, and in specific embodiments of the invention substantially hydrogen-free coatings are produced.
It is a further option for the method to deposit a coating that is a composite of at least first, secondhand third elements and wherein the target comprises said at least first, second and third elements. Alternatively, the coating can be a composite of at least first, second and third elements and the target comprises said at least first and second elements and the method comprises introducing the third element into the coating in a gaseous or liquid form.
It is envisaged that the method of the invention is of application without limit to the choice of target materials. Specifically, the method has successfully been carried out using a target that comprises carbon, producing a coating of a composite comprising tetrahedral amorphous carbon. The target preferably contains, as second element, a metal other than carbon. The target should be electrically conducting, so other target materials may be chosen that are non-metallic, provided that the target is sufficiently conducting to be used as a cathode target in a cathode arc deposition apparatus. Where the second element is a metal it is suitable selected from titanium, nickel, chrome, aluminum, silicon and tungsten. Reference to element is intended to be reference to the element whether present in elemental or ionic or compound form.
In a particularly preferred embodiment of the invention the method comprises depositing a layer of a composite film of carbon and silicon, suitably using a target which contains at least 40% carbon, the remainder being substantially silicon. The composite Sixe2x80x94C film obtained has uses in the semiconductor field. Also the Sixe2x80x94C film obtained can be used for its improved trabelogical properties of reduced stress and high hardness compared to known DLC and DC-based films.
In a further particularly preferred embodiment of the invention the method comprises depositing a layer of a composite film of carbon and aluminium, suitably using a target which contains at least 80% carbon, the remainder being substantially aluminum.
The use of composite targets has the advantage that it is possible according to the invention to deposit coatings that have a high proportion of sp3 bonds. It is preferred that the deposited coating has an sp3 content of at least 60%, more preferably at least 70%, and in specific embodiments of the invention sp3 percentages of 80% and above are achievable.
The invention additionally provides in the first aspect a method of depositing a composite coating of at least first and second elements, comprising:
generating an arc between an anode and a cathode target, wherein the cathode target comprises said first and second elements, so as to generate positive ions of said first and second elements; and
depositing said ions on a substrate to form the composite coating.
The target used in the method can comprise carbon and the composite coating comprise tetrahedral amorphous carbon having an sp3 content of at least 70%.
In a second aspect of the invention there is provided a composite coating comprising tetrahedral amorphous carbon and a metallic element other than carbon, the composite coating having an sp3 content of at least 60%. The sp3 content in preferred coatings is at least 70%.
In embodiments of the invention a composite coating comprises 99.9-80% carbon and 0.1-20% aluminium. These have been found to exhibit particularly desirable properties as more specifically set out in the examples below.
In further embodiments of the invention a composite coating comprises 99.9-40% carbon and 0.1-60% silicon. These have been found to exhibit particularly desirable properties as more specifically set out in the examples below.
An advantage of films of the invention is that they have stress levels that are reduced compared to pure ta-C films, and therefore films of the invention can be deposited at greater thicknesses than pure ta-C films, but retain an acceptable hardness. In terms of their structure, films of the invention retain a significant proportion of the structure seen in pure ta-C films, such as a high level of sp3 bonding. The films of specific embodiments of the invention have additionally been found to exhibit increased adhesion to substrates and to have a good coefficient of friction. The coatings are of use in applications where such properties are sought, in particular on forming tools, lift frames for semiconductor chips, components of moulds especially injection moulds, dies and punches.
In a third aspect of the invention there is provided a substrate coated with a composite coating according to the second aspect of the invention. It is an advantage of the invention that hard thick coatings are obtainable, and the composite coating typically has a thickness of up to 10 microns.
A still further, fourth aspect of the invention provides a target for use in a cathode arc source, comprising a mixture of carbon and a metallic element other than carbon.
The target may comprises carbon and silicon, and may comprise carbon and aluminum. Targets of this composition have been used in specific embodiments of the invention as described in more detail below, resulting in films have advantageous properties. The target may also comprise carbon and another element, such as a metallic element selected from titanium, chromium, nickel and tungsten.
To obtain a target of the invention, a mixture of carbon and the metallic or other element can be sintered, for example in the presence of a binder such as bitumen or tar.
The invention yet further, in a fifth aspect, provides a method of making a target for use in a cathode arc source, comprising:
combining at least first and second target components in powdered and/or finally divided form to produce a mixture of said first and second target components; and
pressing said component to form a target.
The method preferably comprises sintering the mixture of first and second target components at elevated temperature, more preferably at a temperature of 1000xc2x0 C. or higher.
It is optional to include a binder, in which case the method comprises combining the target components in the presence of the binder, such as tar, bitumen, alcohol and mixtures and compositions thereof.
Clean amorphous silicon-carbon (a-SiC) alloy films were thus deposited in specific embodiments of the invention by filtered cathodic vacuum arc technique. The silicon content in the film was determined by X-ray photoelectron spectroscopy (XPS) measurement and found int examples to vary from 2.4 to 48 at. %. Both XPS and Raman measurements showed the existence of amorphous silicon carbide clusters in the film with silicon content between 42 and 48 at. %. With increasing silicon content, the hardness of the film decreases from 62 Gpa to 22 Gpa while the compressive stress decreases from 8.2 Gpa to 2.0 Gpa.
Aluminium-containing tetrahedral amorphous carbon (ta-C:Al) films were similarly thus prepared according to the invention in a filtered cathodic vacuum arc process. Characterization of the films was mainly focused on their mechanical properties and internal stress in terms of film structure and Al content. The film structure was studied mainly by MicroRaman Spectroscopy. The mechanical properties were measured by nanoindentation testing. The internal stress was evaluated with a radius of curvature technique by means of surface profilometry. It was noticed that the internal stress of example films was reduced significantly from 10-12 GPa in the ta-C films to 1-2 GPa in the ta-C:Al films. However, the hardness of films had a drop to around 25 GPa when the Al content in the films was beyond 10 at. %. From the Raman measurement, the ratios of D-peak intensity, full width at half height (FWHH) and peak area to those of G-peak slightly increased with the increase of Al content. This indicates that the amount of sp2 bonding in these films has increased. It appears that the increase of sp2 bonding is not considerable. However, with the further increase of Al in the films, the D-peak developed much faster than the G-peak, meaning that the sp2 component has significantly increased and indicating that in films of the invention Al can effectively be doped in the film as an acceptor, and then the sp3 bonding is maintained, and that Al alloying such as Al4C3 or AIOC can happen when excess Al is introduced into the film. Then Al can exist in clusters in the sp3-dominant carbon network.